The origin of low thermal expansion coefficient and enhanced tensile properties of Invar alloy fabricated by directed energy deposition

Invar alloy has a wide range of applications in different industrial sectors because of its low coefficient of thermal expansion (CTE) below the Curie temperature. Directed Energy Deposition (DED) offers tremendous advantages in fabricating large-size Invar alloy components, but little is known about the relationship between its intrinsic microstructure and properties. This research is designed to explore the relationships between microstructures, mechanical properties, and the CTE of Invar alloy fabricated by wire-arc additive manufacturing (WAAM). The microstructures of the as-deposited (AD) and heat-treated (HT) samples were characterized by columnar γ grains decorated by the primary phase and dispersoids. This results in anisotropic mechanical properties and CTE. The precipitated phases were identified as (Nb, Ti)C (0.5–1 µm) and NbC (30–500 nm). The presence of the precipitated phase with area fraction of 2.4% results in prominent tensile properties with UTS of 530 MPa for the AD sample tested along horizontal direction. The increase in the area fraction of NbC (from 0.446% to 0.798%) after heat treatment is accountable for the further increase in the strength (5–20 MPa) of the HT samples. Adjusting the Ni content of the matrix closer to 36% (from 36.4% to 35.72%) resulted in a lower CTE of the HT samples. As a result, the strength (UTS of HT horizontal sample:540 MPa) and CTE (1.13 ×10⁻⁶ C⁻¹, 0–100 °C) of the HT samples were both superior to those of their counterparts published in the literature. The findings presented in this research pave the way for the fast fabrication of large-sized Invar alloy components through WAAM.